This invention relates to the generation of ultrasonic fields. It is particularly, but not necessarily exclusively, concerned with the generation of such fields for use in the manipulation of particulate matter in a fluid medium, including the removal of particles from a liquid suspension and the segregation of dissimilar particles from a mixture of particles.
Acoustic energy sources have been used to generate progressive and standing waves for a variety of purposes. For example, ultrasonic energy can have an influence on the behaviour of particles suspended in fluids, it being known that particles can be attracted to the nodes of a standing ultrasonic wave. In essence, the attracted particles become concentrated in planes lying normal to the axis of propagation of the standing wave. If the wave is moved along the axis of propagation, the particles can then carried through the fluid while they remain attached to the standing wave.
The detailed theory underlying the observed phenomenon of standing waves and their effect on particles is not fully understood. For example, the factors influencing whether any given particle type tends to accumulate at the "nodes" or at the "antinodes" of a standing wave are unclear. However, this lack of theoretical understanding has no bearing on the practical application of the present invention, and in this specification the terms "nodes" and "nodal planes" are used to include both nodes and antinodes.
When energy is propagated from an ultrasound source through a fluid, the energy level at any point in the fluid will decrease with increasing distance from the source because of attenuation by the fluid. Divergence of the beam accentuates this effect. The acoustic energy propagated by that source is therefore subject to an energy density gradient which is experienced by the fluid as a uni-directional force, in effect a radiation pressure, Such a force can cause the fluid to move away from the radiation source, this movement being referred to herein as acoustic streaming.
If acoustic energy is to be used to control the movement of particles in a volume of fluid, it is more usually the case that a standing wave is employed. Should the stading wave be formed by a normal reflection of ultrasound radiation from a single source, as in the example of U.S. Pat. No. 4280823, it will be apparent that both attenuation and divergence of the acoustic beams will give rise to a radiation pressure throughout the field of the standing wave. The resulting acoustic streaming clearly can have a disturbing effect on any attempt to control the movement of the particles by means of the acoustic forces acting directly on them, and especially if reliance is placed on the acoustic forces to discriminate between different particle types.
By using two opposed ultrasonic transducers to establish a standing wave by the interference between their outputs, it is possible to balance out radiation pressure, at least in substance, although over only a minor part of the distance between the sources at the higher ultrasonic frequency ranges suitable for processing small particles. Thus, for a standing wave in water at 20.degree. C., the following Table shows the total working distance available in mm within three different tolerance levels of imbalance for different frequencies, ignoring the effects of divergence:
TABLE 1 ______________________________________ Total working distance available (mm) Percent imbalance Frequency MHz 5% 2% 1% ______________________________________ 0.3 11,400 4,400 2,200 1.0 1,000 400 200 3.0 114 44 22 10.0 10.3 4.0 2.0 ______________________________________
Clearly, it would be desirable to avoid generating radiation pressure within the liquid, or at least to keep such pressures sufficiently low to prevent any significant acoustic streaming, in order to have the maximum volume of the acoustic field available for particle manipulation, such as a separation process. This would dictate the use of very low frequencies because, as the table shows, the working distance can be increased considerably. However, high frequencies provide a more efficient separation process in that particles then adhere more firmly to the nodes. It is an object of the present invention to mitigate the problem posed by the streaming phenomenon and permit effective use of high frequencies.